Optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) have been used as power amplifiers, repeaters and preamplifiers in lightwave systems and have been responsible for significant improvement in the performance of long-distance transmission systems, networks, CATV distribution and the like. A number of types of optical amplifiers have been developed. One class of optical amplifiers is rare-earth doped optical amplifiers, which use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped optical amplifiers have proven to be particularly attractive because they are operable in the spectral region where optical loss in the silica core is minimal. Also, the erbium-doped optical amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. Important features of these amplifiers include high gain, low noise (near quantum limit) and high saturated output power.
Optical amplifiers are designed by considering a number of parameters including gain, output power, compression (i.e. gain saturation), and noise performance. Noise performance is typically measured by the noise figure, which is defined as the signal-to-noise ratio at the input of the optical amplifier divided by that at the output. When optical amplifiers are used as repeaters, they should operate with very low noise figure and high output power in order to maximize the distance between adjacent repeaters in the lightwave system.
Initially, optical systems utilized relatively simple single-stage amplifiers in combination with ancillary passive components such as isolators, pump multiplexers, and power monitors, which are attached to either end of the fiber amplifier. It has been found, however, that this arrangement not only leads to rather stringent design and fabrication tolerances for a high-performance optical amplifier, but it prevents the full exploitation of erbium-doped fiber amplifiers.
Optical amplifier arrangements are known which use multiple EDFA stages to improve the performance characteristics of the optical amplifier. In such an arrangement, two or more separate stages of amplification are separated by passive optical components. FIG. 1 shows an example of a known two stage optical amplifier. The first stage of amplification 12 is coupled to a second stage of amplification 14 through a passive, optically lossy element 16 such as an isolator or attenuator. The first stage 12 includes an erbium doped amplifying fiber 18 coupled via a coupler 20 to both an input port 22 for receiving a signal that is to be amplified and to a pump port 24 for receiving energy from a laser diode pump 26. The second stage 14 of the multistage amplifier includes a second erbium doped amplifying fiber 14 coupled via a coupler 28 to both an output port 30 for providing a signal which has been amplified and to a pump port 32 for receiving energy from a laser diode pump 34.
The output port 36 of doped fiber 18 is coupled to the input port 38 of doped fiber 19 via the passive, optically lossy optical element 16. The optical element 16 is generally coupled to the optical fibers of the first stage 12 and the second stage 14 by fusion, by splicing or by other means. The coupler 20 is constructed to couple both the input signal received by input port 22 and the pump signal received by pump port 24 to doped fiber 18. In a similar manner, coupler 28 is constructed to pass the amplified signal from the multistage amplifier to the output port 30 and to couple the pump signal received by pump port 32 to amplifying fiber 14.
In order to achieve an optical amplifier with a low noise figure and a high output power, the first stage of amplification 12 is generally optimized to provide a low noise figure at all wavelengths and low to moderate output power. The second stage of amplification 14 is generally optimized to provide high output power and efficiency at the expense of an increased noise figure. If more than two amplification stages are employed, the intermediate stages are generally optimized to provide an intermediate noise figure and an intermediate level of output power.
One problem with an optical amplifier that comprises multistage EDFAs is that because they employ a multitude of components they can be difficult to assemble and require a large number of splices that contribute to loss that reduces the overall gain supplied by the amplifier. Moreover, as the number of stages increase, the amount of space the amplifier occupies can be come unduly large. Accordingly, it would be desirable to provide a multistage optical amplifier that overcomes the aforementioned deficiencies.